Don't know what version your're running;
PIX/ASA 7.x and later
Enter the vpn-idle-timeout command in group-policy configuration mode or in username configuration mode in order to configure the user timeout period:
hostname(config)#group-policy DfltGrpPolicy attributes
hostname(config-group-policy)#vpn-idle-timeout none
Configure a maximum amount of time for VPN connections with the vpn-session-timeout command in group-policy configuration mode or in username configuration mode:
hostname(config)#group-policy DfltGrpPolicy attributes
hostname(config-group-policy)#vpn-session-timeout none
So, the vpn-session-timeout command can be used to do that.
Cheers,
rsivanandan